I am hibernating my T.graeca ibera and marginata in a refrigerator, as I live in a similar (even warmer) climate to yours. I'm in the Imperial Valley of CA. Check out tortoiseman.com web-site, Dean provides some great information on hibernating. Do what he says, don't deviate significantly, he has been long term successful. I found a cool digital temp/hygrometer at a well known electronics shack for about $19.00, so I can see the temp at a glance.
Tortoises have been hibernating for years, so provide the basics and you should do fine. Do your research, the above source and perhaps CTTC also. Good luck, Vic Herrick P.S. I would hibernate small ones not more than 8-10 weeks, adults 14 weeks or so. Assuming they enter hibernation in prime health.